Historically, the transmission of digital images over computer networks has long been a part of modem informational technology. Special graphic formats have been developed to provide the ability to store digital images on storage media and to transmit them over computer networks. These graphic formats typically provide for compressing the image so that the image takes up less space when stored and/or takes less time to transmit over a network. Techniques have been sought and developed for the efficient compression of images in a progressive manner; that is, first an image with limited resolution is displayed, then the same image is displayed in successive stages with progressively greater resolution. Some graphic formats provide for progressive image transmission but typically for color images only. For instance, one format is based on a standard format from JPEG (Joint Photographic Experts Group). The JPEG format allows progressive transmission of color images up to about 16 million colors. Another format is the interlaced GIF (Graphics Interchange Format) created by Compuserve, Inc. The GIF format also allows progressive transmission of color images of 256 colors and even bi-level or bi-tonal images, but the compression methods do not provide for a very high level of compression, thus causing network traffic and delays when compressed images are transmitted over a network.
A very high level of compression for bi-level images is reached in Group 4 compression based on the ITU-T Group 4 specification from the ITU (International Telecommunication Union), formerly CCITT (International Consultative Committee for Telegraph and Telephone). The Group 4 compression scheme is used, for instance, in the TIFF (Tagged Image File Format) graphic format developed by Aldus Corporation and widely used for storing documents as digital images. The Group 4 compression scheme does not allow for progressive transmission but only for sequential transmission of lines in an image. This is due to a restriction on the Group 4 compression scheme whereby the compression of each row or line of the image currently being coded is based on the contents of the previous row (the reference line). Thus, the Group 4 coding scheme requires the compression of sequential lines in an image.
Therefore, one object of the invention is to provide methods for the high compression of bi-tonal and other images in a progressive manner and for on-demand enhancement of the displayed images. Another object is to provide a compression technique suitable for efficient, progressive display of images over a global network. Another object is to provide an interlacing approach to progressive compression based on multiple compression methods, including a modified Group 4 method, and adaptable to different levels of progressive compression.